Radial fans can in principle be classified into two different categories: one group being formed by radial fans with spiral housing and the other group by free-running radial fans.
The spiral housing of the radial fans in the first group has two objects to fulfill. It collects the outflowing air from the impeller, guides it to a common discharge point and converts a portion of the kinetic energy (dynamic pressure) into pressure energy (static pressure) through the steady cross-sectional expansion in the flow direction (diffusion effect). In free-running radial fans, the air in the impeller is diverted in a radial direction and flows out there at the blade discharge diameter of the impeller. To increase the degree of static efficiency, diffusers are already known where the cover disc and bottom disc of the impeller have outer edge areas protruding from the blade discharge diameter.